Piezoelectric extension actuator

ABSTRACT

A piezoelectric expansion actuator for d33 piezoelements, which allows vibrations in structures to be suppressed. An expansion actuator ( 1 ) includes a piezoelectric stack ( 2 ), which consists of d33 piezoelectric elements and is arranged between output elements ( 4 ), which are attached to the surface of the structure ( 7 ). The invention applies to a piezoelectric extension actuator, which is used to control vibrations in structures. Alternatively, in order to damp vibrations between the main gearbox of a helicopter rotor and the cellular structure of the cockpit, a power application point of the output element ( 18, 19, 180, 190; 35, 36 ) is arranged at a distance from the corresponding end plate of the piezoelectric stack ( 22, 220; 31, 32, 33 ) in the axial direction (X).

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German Application Nos. 100 49176.6, filed Oct. 5, 2000 and 101 39 686.4, filed Aug. 11, 2001 andInternational Application No. PCT/EP01/11360, filed Oct. 2, 2001, thedisclosures of which are expressly incorporated by reference herein.

The invention relates to a piezoelectric expansion actuator.

The use of d31 piezoplates or d31 piezosegments is known for the purposeof vibration control and to influence vibrations in structures. d31piezoplates take advantage of the elastic transverse contraction of thepiezoelectric material. Several piezoplates or piezosegments will bedescribed as piezoelectric stacks. A piezoelectric stack consists ofseveral, but at least 2 piezoelements. With the above d31 piezoelements,for example, expansions are introduced into carrier structures forhelicopter transmissions so as to suppress the transmission of bodysound onto the helicopter cell. In doing so, the d31 piezoelements areintegrated in accordance with their expansion direction, which actsparallel to the surface of the d31 elements, into the surface of thecarrier structure across a large surface, e.g. through an adhesiontechnique.

By contrast, the expansion in the familiar d33 piezoelements actsperpendicular to the surface of the elements because d33 piezoplatestake advantage of the expansion of the piezoelectric material in thedirection of the applied field.

German Patent No. 198 13 959 A1 describes a device for body soundsuppression that more effectively reduces the transmission of equipmentvibrations and oscillations through a carrier structure onto a cellularstructure of a cockpit in a simple construction and at relatively lowintegration complexity. German Patent 198 13 959 A1 provides that thesound suppression device includes at least one piezoactuator, whichintroduces the oscillations into the carrier structure in order to blockthe body sound transmission path onto the insulating structuresubstantially and to compensate acoustic excitation by use of theexisting and excited system dimensions of the sound generator moreeffectively. This technical idea is not limited to use in helicoptermanufacturing. It can be employed in all areas of mechanical engineeringwhere a device for body sound suppression becomes necessary.

Contrary to other familiar expansion actuators, the piezoactuator doesnot implement the application of power onto the carrier structure atonly points, but rather across a relatively large surface of the carrierstructure. The carrier structure can be arranged for example between themain gearbox of a rotor and a cellular structure of the cockpit of ahelicopter. In this case, the carrier structure would be one or morestruts (also called gear struts). The piezoactuator is largely arrangedalong the entire circumference of the strut and exhibits a definedexpansion in the axial direction of the strut. Forces are introduced bythe piezoactuator pursuant to German Patent DE 198 13 959 A1 via itssurface.

The efficiency of power application is limited by the effective surfaceof the strut that is to be covered.

The invention is based on the development of a piezoelectric expansionactuator for d33 piezoelements, with which vibrations can be suppressedin structures, and furthermore of considerably increasing the efficiencyof power application of a piezoactuator despite the contrary tendency ofdecreasing construction volume of the piezoactuator.

A solution pursuant to the invention is based on the fact that a d33piezoelement in the form of a stack is clamped into a mechanical frame,which is fastened to the surface of the structure. Apart from a highlyspecific, mechanical power, the expansion actuator also achieves goodefficiency. Also beneficial is the application of mechanical pre-stressthat is integrated in the actuator which allows critical tension strainto be avoided for the piezoelements. Optionally, devices can beintegrated in the frame with which stroke speed transformations orstiffness transformations can be beneficially achieved.

In another solution pursuant to the invention, the efficiency of powerapplication for the piezo actuator can be improved by considerablyincreasing the distance between the resting areas of two output elementsof a piezoactuator and a corresponding end plate of the piezoelectricstack in the axial direction towards the strut end. The output elementsof the mechanical frame form the power transmission device from thepiezoactuator to the strut. The thus considerably enlarged strutdistance between the resting areas of the two output elements exhibitsless stiffness, consequently leading to an expansion of this strutsection with less force required than in a comparable configuration of apiezoactuator where the distance of the resting surfaces of the outputelements largely corresponds to the length of the piezoelectric stack.The piezoelement also uses the d33 piezoelectric elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Based on the drawing, exemplary designs of the invention are explainedin more detail in the following:

FIG. 1 is a section view of an expansion actuator,

FIG. 2 is an expansion actuator with stroke speed transformation,

FIG. 3 shows an alternative version of an expansion actuator with strokespeed transformation,

FIG. 4 shows a diagram of a strut with axially spaced output elements ofa piezoactuator,

FIG. 5 illustrates a sectional view of a strut with collar-shaped outputelements of a piezo actuator, and

FIG. 6 shows an alternative design of the output element with recesses.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The expansion actuator 1 shown in FIG. 1 is rigidly fastened to thesurface of a structure 7 and consists of a d33 piezoelectric stack 2,two end plates 3, two output elements 4 and a prestress element 6.

The d33 piezoelectric stack 2 is arranged in its mechanical frame suchthat its expansion direction runs parallel to the surface of thestructure 6 in which the expansion actuator 1 transmits itspiezoelectrically generated expansions. The d33 piezoelectric stacktakes up ⅓ of the material volume of a d31 piezoelectric stack forequivalent active expansions.

In the design in FIG. 1 the mechanical frame is formed by the two outputelements 4, which are rigidly attached to the structure 6. To accomplishthis, the output elements 4 are attached in such a way to the surface ofthe structure 7 that their output surface 5 is aligned parallel to therespective end plate 3 of the piezoelectric stack 2. The output elements4 can be fastened to the structure 7 by means of familiar attachmenttechniques, for example by gluing.

The output elements 4 can be adjusted on their attachment surface tovariously bent or plane structural surfaces. In the design shown thestructure is a pipe with a circular concave surface. The length of thepiezoactuator corresponds to the length of the strut section that issupposed to be expanded.

The piezoelectric stack 2 is seated between its two end plates 3 andheld in place with a prestress element 6 at mechanical precompressionstress. Possible damaging tensile loads acting upon the expansionactuator 1 are compensated with this precompression stress and can thushave no effect on the piezoelectric stack 2.

The prestress element 6 can be implemented for example with one or moremechanically acting tension springs—as indicated symbolically in thedesign in FIG. 1. It is also possible, however, to design the end plates3 as elastic plates and to insert the piezoelectric stack 2 withcompressed end plates 3 into the mechanical frame at precompressionstress.

The expansion actuator 1 with stroke speed transformation shown in FIG.2 corresponds to the previously described design except for thefollowing deviations. The end plates 3 of the piezoelectric stack 2 arenot connected directly with the output surfaces 5 of the output elements4, but rather by means of an elastic pressure web 8, and the outputelements 4 comprise an inwardly one-sided open slot 9 that runs parallelto the surface of the structure 7. Furthermore, the two output elements4 are rigidly connected with each other by means of a non-expandingsupport bar 11, which engages in the free end 10 of the output elements4. In place of a support bar 11 two parallel support bars 11 that arearranged on either side of the piezoelectric stack 2 can be used, as isrevealed in FIG. 2 with a support bar 11 shown in the drawing.

On each output element 4, the elastic pressure webs 8, slots 9 andsupport bars 11 form three joints “a”, “b” and “c” about which the leversections of the output elements 4 can rotate and generate a stroke speedtransformation in the expansion actuator 1.

FIG. 3 depicts an expansion actuator 1 with stroke speed transformationin an alternative design version compared to FIG. 2 of the outputelements 4 and joints “a”, “b” and “c”. The effect of lever sectionsabout the joints “a”, “b” and “c” in principle corresponds to thepreviously described design in FIG. 2.

The output elements 4 here are designed with a lever 12 that is seatedin joint “a”. The joint “b”, in which the piezoelectric stack 2 engageswith an output web 14, is arranged on the lever 12 with a first leversection 13 at a distance to joint “a”.

Joint “c” is arranged on the lever 12 with a second lever section 15 ata distance to joint “b”. Joint “c” engages in the support bar 11.

FIG. 4 shows a diagrammatic image of a strut 16. The strut can be, forexample, a steel pipe with a fastening loop that is welded onto eachend. Such a strut is used, for example, in a quadruple setting in orderto connect the main gearbox of the rotor of the helicopter with thecellular structure of the cockpit of the helicopter. The main gearbox ishereby located above the ceiling of the cellular structure of thecockpit. The two components are connected in 4 locations by a strut 16,respectively. The main gearbox of the rotor is one of the main sourcesof noise generated in the cockpit. Since the strut 16 is seated on theinterface between the main gearbox and the cellular structure, it isuseful if elastic dimensional changes are generated on the strut, whichcan largely compensate the forces introduced via the strut. This iseffected, for example, through a controlled dimensional change(expansion or contraction) of the strut 16 in the axial direction X. Thecontrolled elastic dimension change is implemented with thepiezoactuator 17, which initiates a dimensional change, particularly achange in length in the axial direction X in a certain section D of thestrut. In the strut 16 pursuant to FIG. 4 additionally there are twooutput elements 18, 19 arranged per piezoelectric stack. The outputelements 18, 19, however, are not connected directly behind the endplate 20, 21 of the piezoelectric stack 22 with the surface of the strut16, but the resting surfaces 23, 24 of the output elements 18, 19 arearranged at a distance to the end plate 20, 21 of the piezoelectricstack 22 in the axial direction X. The piezoelectric stack 22 does nothave to rest directly on the surface of the strut. The force that isgenerated by the piezoelectric stack 22 is introduced into the strut 16on the resting surface 23, 24. This force effects an elastic dimensionalchange in a section D of the strut 16 between the two resting surfaces23, 24.

The section D along the strut circumference includes the correspondingsectional spatial structure of the strut. The elastic dimensional changecompensates the vibration force in the strut 16, specifically in thearea of the interface of strut and cellular structure.

The piezoelectric expansion actuator 17 is formed by d33 piezoelectricelements, which are arranged in a piezoelectric stack 22. The two endsof the piezoelectric stack 22 are limited by the end plates 20, 21. Theoutput elements 18, 19 are arranged on the end plates 20, 21. The powerapplication point of an output element 18, 19 on the strut 16 isarranged at a distance from the end plate 20, 21 in the axial directionX towards the fastening loop 160, 161. Gaining such a distance isassociated with gaining a lever arm that engages on both sides of theend plates of the piezoelectric stack. One lever arm E, F each is formedby an output element 18, 19. The lever arms E, F increase the section Dby their length since originally section D corresponded only to thelength of the piezoelectric stack.

The tensile force, for example, that is generated in a selection of thepiezoelectric expansion actuator is introduced into the strut via theoutput surface of the output element. The section D located between theoutput surfaces 23, 24 of the strut 16 is thus exposed to a controlleddimensional change in an axial direction X. This change represents anelastic dimensional change. Compared to the previously describedsolution, this alternative solution takes advantage of the lowerrigidity of an enlarged strut section. This increases the efficiency ofpower application of a piezoactuator 17 considerably. It is, therefore,possible to use a substantially smaller piezoelectric stack withouthaving to accept an efficiency loss.

Multi-axis influencing of the dimensional change of the describedsection D of the strut 16 can be controlled as a function of thepiezoelectric stack's configuration along the circumference of thestrut.

The arrangement shown in FIG. 4 can be also designed in the inside of atubular strut.

As FIG. 4 also shows, this elastic dimensional change of the section Dis controlled with a control unit 25. In the area of each output element18, 19, preferably in the vicinity of the resting surface 23, 24, thecontrol unit comprises a sensor 26, which determines a signal from thequantitative value of the existing vibrational forces and supplies it tothe control device. The control device 25 regulates the piezoelectricstack 22 such that a force is generated by the piezoelectric stack andis introduced into the strut 16 via the output elements 18, 19 so as toaffect an elastic dimensional change of the strut section D.

The distance D between the resting surfaces 23, 24 can be implemented ina variably adjustable manner by designing at least one lever arm E, Fsuch that it can be enlarged and reduced.

The above explanations apply similarly to the piezoactuator 170 in FIG.4 with the output elements 180, 190 and the piezoelectric stack 220 withthe end plates 200, 210.

FIG. 5 shows a sectional view of a design for a strut 30, wherein thetwo fastening loops on the ends of the strut 30 are not depicted.

FIG. 5 depicts three piezoelectric stacks 31, 32, 33, which form apiezoactuator 34 along with the output elements 35, 36. Thesepiezoelectric stacks 31, 32, 33 are offset from each other, for example,by 120°. Each piezoelectric stack can be arranged at a distance from thesurface of the strut. The stacks, however, can also be arranged on thesurface of the strut 30. The first version is shown in the example.

The axial axis of the piezoelectric stack is aligned in the direction ofthe axial axis X of the strut 30. Each piezoelectric stack is arrangedbetween two output elements 35, 36. The two output elements 35, 36engaging one piezoelectric stack 31, 32, 33, respectively, contain eachan annular socket 37, 38, which encloses the strut 30 in an interlockingand non-positive manner along its circumferential surface. Extendingfrom the annular socket 37, 38 the output element 35, 36 opens up in abell-shaped manner like a collar, which is arranged at a distance fromthe strut starting from the edge of the annular socket to its annularedge. This shape is described as an annular collar 39, 40. On the edge41 of the collar 39 rests one end of the piezoelectric stack 31, 32, 33,respectively. The other end of the three piezoelectric stack 31, 32, 33is respectively located on the edge 42 of the collar 40.

The annular socket 37, 38 of the collar 39, 40 exhibits sufficientrigidity and firmness that corresponds to a resting surface 370, 380which is connected with the surface of the strut 30 in thecircumferential direction in an interlocking and non-positive manner.The forces generated by the piezoelectric stacks 31, 32, 33 areintroduced via the resting surfaces 370, 380. Such a design for anoutput element 35, 36 permits action in a variety of spatial axes. Hencemore variable design possibilities exist for introducing power into thestrut 30. The forces and bending moments that are introduced into thestrut 30 can be used to effect an excursion in the longitudinal (axial)direction, a lateral bending excursion in any random direction and alsotorsion of the strut 30.

This elastic dimensional change also affects a structural region in thestrut 30 along section D.

By using at least two piezoelectric stacks that are arranged around thestrut, the strut can be displaced in the longitudinal and lateraldirections through appropriate selection of the individual piezoelectricstacks. A suitable control or regulating device is not shown in FIG. 5.

It is also possible to introduce torsional forces by inserting thepiezoelectric stack at an angle, i.e. a configuration of at least onepiezoelectric stack that is tilted in relation to the longitudinal axisX of the strut 30.

FIG. 6 depicts another possible design of an output element. It is shownas a single output element 360 without strut and without piezoelectricstack. The output element 360 is guided in the direction of the axialaxis X of a strut and fastened to the surface of the strut by means ofits annular socket 390. A collar 400 is incorporated on the annularsocket. This collar 400 contains recesses 401 so that weight of theoutput element can be saved. The collar 400 includes for example threearms, which can be arranged at an angle of 120° in relation to oneanother. These three arms of the collar 400 are connected and limited attheir ends by a ring 420. This ring 420 forms the edge of the collar400. One end of a piezoelectric stack is arranged on the edge of thecollar, respectively.

Pursuant to another design (not shown), it is also possible to divide anoutput element 360 partially into sectional output elements. Toaccomplish this, sectional output elements are arranged in anon-positive manner with each other in one direction along thecircumference of a strut in segments and are connected. In the viewingdirection of the X-axis, an output element can be divided intoindividual (wedge-shaped) segments, which are arranged around theX-axis. The output element is thus composed in segments of sectionaloutput elements. The output element pursuant to FIG. 5, for example,could be composed of three sectional output elements in the case ofthree piezoelectric stacks. This configuration of sectional outputelements is an easy option for retrofitting a strut on the helicopterthat has already been installed.

Such a configuration makes it possible to reduce the vibrationsgenerated by the main gearbox in relation of the cellular structure ofthe cockpit efficiently for the pilot and noticeably for the passengers.

1. A piezoelectric expansion actuator for reducing vibrations in astructure, said actuator comprising: a pair of output elements attachedto a surface of said structure; and a piezoelectric stack including aplurality of d33 piezoelectric elements and first and second end platespositioned on respective first and second ends of said piezoelectricstack; wherein, surfaces of said output elements which are positioned onsaid structure are arranged in a longitudinal direction of saidstructure relative to corresponding ones of said first and second endplates of the piezoelectric stack whereby, when said piezoelectric stackintroduces power into said structure, a section of power applicationbecomes larger than a length of the piezoelectric stack in order toeffect a stiffness transformation; mechanical precompression stress isapplied on the piezoelectric stack by means of a prestress element; andthe prestress element comprises at least one mechanical tension spring.2. The expansion actuator according to claim 1, wherein the prestresselement comprises elastic end plates, which define the extent of thepiezoelectric stack that is inserted in the output elements by pressure.3. The expansion actuator according to claim 1, further including anintegrated means for effecting a stroke speed transformation.
 4. Theexpansion actuator according to claim 3 wherein, said means foreffecting a stroke speed transformation comprises, in the outputelements, an inwardly one-sided open slot that runs parallel to thesurface of the structure and an elastic pressure web, respectively,which are arranged between the end plates of the piezoelectric stack andthe output surfaces of the output elements.
 5. The expansion actuatoraccording to claim 3 wherein; said means for effecting a stroke speedtransformation comprises, in the output elements, an articulating lever;behind a first lever section, the piezoelectric stack with an output webengages the lever in an articulating manner; and behind a second leversection the support bar engages the lever in an articulating manner. 6.A piezoelectric expansion actuator for reducing vibrations including atleast one piezoactuator arranged on a strut, wherein said strut connectsa main gearbox of a helicopter rotor with a cellular structure of acockpit, each of said piezoactuator actuators comprising a pair ofoutput elements attached to a surface of the strut and at least onepiezoelectric stack made of d33 piezoelectric elements positionedbetween said pair of output elements, wherein resting surfaces of saidoutput elements are arranged on the strut at a distance in relation torespective end plates of said piezoelectric stack in the axial directionwhereby when said piezoactuator introduces a control force in onesection of the strut for purposes of elastic dimensional change of saidsection of the strut, an area of section of power application isincreased when compared to an area covered by said piezoelectric stackin order to thereby reduce stiffness of the section of the strut.
 7. Theexpansion actuator according to claim 6, wherein the output elementforces a lever arm (E, F) extending said output elements configurationon an end plate up to a point of said power application.
 8. Theexpansion actuator according to claim 6, wherein a distance (D) of theresting surfaces of the output elements can be adjusted variably.
 9. Theexpansion actuator according to claim 6, wherein the output elements aredesigned as collars.
 10. The expansion actuator according to claim 9,wherein, between the collar-shaped output elements, the piezoelectricstacks are arranged offset from each other by 120°.
 11. The expansionactuator according to claim 9, wherein the collar of the output elementcontains recesses.
 12. The expansion actuator according to claim 6,wherein the output elements are composed of sectional output elements ina circumferential direction of the strut so that retrofitting of analready installed strut is possible.